Development of microbial carbonates in the Lower Cretaceous Codó Formation (north‐east Brazil): Implications for interpretation of microbialite facies associations and palaeoenvironmental conditions

Abstract

The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time. Additionally, microbialites have a demonstrated economic potential to serve as excellent hydrocarbon reservoirs. The Lower Cretaceous Cod o Formation, located in the Parnaiba Basin of north-east Brazil, comprises a unique stratigraphic sequence of up to 20 m thick, well-preserved carbonate microbialites. Deposited in a continental basin during the initial break up and separation of South America from Africa in the Early Cretaceous, this lacustrine carbonate sequence provides an excellent example to investigate the palaeoenvironmental conditions controlling microbialite facies development. Based on macroscopic and microscopic observations of outcrop and drill core samples, four microbialite facies (stromatolite, lamina, massive and spherulite) were defined and distinguished by textures and microbial fossil content. Changes in facies type are related to alternating palaeo-water depths, as reflected by 87 Sr/ 86 Sr cycles resulting from fluctuations in the sources of meteoric water. Clumped isotope measurements of stromatolitic fabrics yield precipitation palaeo-temperatures with an average value of 35°C. The d 18 O values of bulk carbonate (À6Á8 to À1Á5& Vienna Pee Dee Belemnite) imply precipitation from water with calculated d 18 O values between À1Á6& and 1Á8& Vienna Standard Mean Ocean Water, reflecting precipitation from variably modified meteoric waters. The d 13 C values of bulk carbonate (À15Á5 to À7Á2& Vienna Pee Dee Belemnite) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter. Combined, the data indicate that the evolution of the Cod o Formation occurred in a closed lacustrine palaeoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations. The results provide new insights into palaeoenvironmental settings, biogenicity and early diagenetic processes involved in the formation of ancient carbonate microbialites and, by extension, improve the knowledge of the reservoir geology of correlative units in deep waters offshore Brazil. 155

Lacustrine stromatolites: Useful structures for environmental interpretation – an example from the Miocene Ebro Basin

“…Other examples have been reported from the Miocene of New Zealand (Lindqvist, ), where they are associated with largely siliciclastic lake deposits, the Oligocene–Miocene of the French Central Massif (Bertrand‐Sarfati et al ., ; Roche et al ., ) and the Eocene Green River Formation in Utah, USA (Awramik & Buchheim, ; Chidsey et al ., ; Della Porta, ). Examples of Pre‐Cenozoic lacustrine stromatolites are scant with the exception of studies from the Cretaceous of the Gyeongsang Basin in Korea (Woo et al ., ; Nehza et al ., ) and the Cameros Basin in Spain (Suarez‐Gonzalez et al ., ), from the lower Cretaceous of the Codó Formation in Brazil (Bahniuk et al ., ), from the Upper Jurassic–Lower Cretaceous of Wessex Basin in the UK (Gallois et al ., ), from the Triassic of the Arnstadt Formation in Germany (Arp et al ., ) and from the late Archean in Western Australia (Awramik & Buchheim, ).…”

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…Microbially created carbonate minerals may also result from the metabolic production of CO 2 ; these are best identified by a combination of textural, compositional and stable isotope studies. It is now recognized that some carbonate reservoirs only exist owing to microbial activity (Lipinski et al 2013;Bahniuk et al 2015).…”

Stable and clumped isotopes in desert carbonate spring and lake deposits reveal palaeohydrology: A case study of the Lower Jurassic Navajo Sandstone, south‐western USA

“…This difference is consistent with enhanced surface evaporation in closed lake basins after groundwater emergence from the springs (e.g. Bahniuk et al ., ; Horton et al ., ). Although this overall enrichment is less substantial than observed in some evaporative carbonates (e.g.…”